The manufacture of microelectronic devices (semiconductors based on silicon or compound semiconductors, integrated circuits, logic, memory, etc as well as magnetic recording media, displays, LEDs, OLEDs, MEMS, MEOMS, Rf devices, etc.) is progressing toward needing scCO2-based processes to deliver performance-enabling manufacturing technology in a wide range of process applications (cleaning, stripping, etching, polymer-layer deposition, photoresist deposition, photoresist manipulation, lithographic image development, CMP, metal layer deposition, seed layer deposition, barrier layer deposition, and metal fill steps). A broad family of these needed applications is the deposition of thin and often conformal films onto surfaces in the manufacture of microelectronic devices, or specifically semiconductors.
High purity metals and metal oxides are of great importance to the microelectronic industry, as are the deposition processes that can provide thin and conformal films. As minimum feature sizes shrink and certain device structures require deposition into high aspect ratio structures, conventional physical vapor deposition (PVD) processes fail to deliver needed film qualities. While chemical vapor deposition (CVD) can deliver enhanced conformality, the need for precursor volatility can limit the metal precursor sources. Additionally, it is not believed that CVD techniques will meet the stringent requirements of the 45-nm node and beyond.
Atomic Layer Deposition (ALD) is a heavily evaluated technique that has not yet broadly impacted the microelectronics industry. It is believed that ALD can deliver the needed metallic film qualities at the 45-nm node and beyond. However, this technology is not without limitations. ALD is a cyclical process where alternating cycles of chemical reagents are applied to a substrate under vacuum. During each so-called half cycle, a monolayer or a fraction of a monolayer is chemisorbed to the substrate surface and a subsequent redox reaction takes place. Because the reaction chemistry is controlled at the surface, the films are typically very conformal but application of films of appreciable thickness is notably slower than conventional processes. Additionally, ALD being a vacuum process presents an integration challenge for the adoption of porous low k materials in that barrier application using ALD can intercalate barrier materials into the pore structure of the insulator layers.
In the field of semiconductor device manufacturing, deposition processes can be characterized by sidewall step coverage and aspect ratio. Sidewall step coverage is defined as the ratio of a layer's thickness on the sidewall of a feature to that on a horizontal surface adjacent to the feature and is generally expressed as a percentage. Aspect ratio is defined as the height of a feature (e.g., a via or trench) versus the width of the feature. A drawback of the conventional copper interconnect dual damascene and single damascene process is the low sidewall step coverage obtained for Ta and TaN copper interconnect barrier layers formed using Physical Vapor Deposition (PVD) techniques. For high aspect ratio via and trench features (e.g., aspect ratios of 4:1 and greater), the sidewall step coverage obtained with PVD techniques is typically around 10%. Atomic Layer Deposition (ALD) has been investigated to overcome this problem, but is inherently slow.
Metal deposition processes also need to be flexible to the application of new materials needed to meet the changing needs of the microelectronics industry. Both CVD and ALD rely heavily on the availability of volatile precursors. What is needed is a technique that meets the stringent requirements of the 45-nm node and beyond and also provides few limitations in terms materials and processing parameters.
Liquid and supercritical CO2 have been disclosed for deposition of a variety of materials to substrates including semiconductor wafers. In U.S. Pat. No. 6,001,148 DeSimone et al provide a method for spin-coating a liquid CO2 soluble photoresist onto a semiconductor wafer. In U.S. Pat. No. 6,083,565 Carbonell et al show the use of liquid CO2 in meniscus coating a variety of substrates with CO2 soluble polymers. In U.S. Pat. No. 6,165,559 McClain et al show the use of liquid and supercritical CO2 for application of predominantly polymeric films to solid substrates. All of these process methods exploit the low or nonexistent surface tension and extremely low viscosity of dense CO2 to provide superior film qualities.
In U.S. Pat. No. 5,789,027 Watkins et al. provide a method for applying metallic films to substrates including semiconductor materials using supercritical CO2 to chemically deposit materials onto substrates. In the described process a metal organic precursor material is added to supercritical CO2 and exposed to a target substrate. For film growth to occur a chemical reagent, typically a reducing gas, is added to the supercritical CO2 composition to drive the chemical reaction that results in metal deposition. To specifically deposit metal on the substrate, the substrate is typically heated in a so-called ‘cold-wall’ reactor design. This method takes advantage of the ideal wetting properties (no surface tension, low viscosity) of the fluid media to provide high quality films. However, unlike ALD where reactions are self limiting, the chemical kinetics of this deposition method is not easily modeled or well understood. Furthermore, heat transfer from the substrate, which may be in excess of 200° C., to the fluid, at ideally a substantially lower temperature, is dynamic and substantially influences film properties. Additionally, this method will also result in deposition of metal into porous substrates limiting its integration capacity for porous low k materials. Finally, this process method has not demonstrated the ability to deposit contiguous conformal films down to and below 10 nm for barrier layer deposition at the 45-nm node and beyond.
What is needed is a method that exploits the fluid properties of liquid and supercritical CO2. What is needed is a method that is capable of preventing the intercalation of metal into porous substrates. What is needed is a method where the deposition of the metallic film is not complicated by heat transfer issues. What is needed is a method where the chemical kinetics can be understood and therefore controlled. What is needed is a method where the precursor material (organometallic) is separate and independent from the reaction step.